Field emission cathode and field emission display

ABSTRACT

A manufacture and methods are provided for a field emission cathode and field emission display comprising a conjugated polymer material. The manufacture of the invention comprises a conjugated polymer material, which may include substituted polythiophene, polyalkylthiophene, and poly-3-octylthiophene. A polymer material layer may be formed by distributing a conjugated polymer material and a solvent onto a substrate. The solvent may be evaporated under a vacuum. The polymer layer may be molded to include projections to promote field emission. Additionally, the polymer material may be doped with an electron donor material. Methods according to the invention include the steps of forming a polymer layer comprising conjugated polymer material on a substrate, distributing a polymer solution including a solvent onto the substrate, evaporating the substrate, and shaping the surface of the polymer layer by use of a mould.

CROSS REFERENCE TO RELATED APPLICATION

This application is the United States national phase of PCT ApplicationNumber PCT/GB99/0076; filed Mar. 19, 1999 which claims priority under 37CFR 119(a-d) to United Kingdom Application No. 9805271.5, filed Mar. 13,1998.

The present invention is concerned with field emission electrodes andwith field emission displays.

A great deal of research effort has be devoted in recent years todeveloping a display which can replace the conventional cathode may tube(CRT). Shortcomings of the CRT include its weight and bulk, and also itsrequirement for relatively high input power. The CRT is also unsuited touse in large area displays eg. in stadia and in small displays forlaptops, watches etc.

The liquid crystal display (LCD) provides an alternative which is notsubject to some of the disadvantages of the CRT. It em be manufacturedin flat panel format, and used both in miniature displays an in largerdisplays, eg. in wide format television sets. However, LCD displayssuffer from disadvantages of their own, particularly with regard todisplay brightness.

A promising alternative to both LCDs and CRTs is the field emissiondisplay (FED). FEDs offer the prospect of flat panel displays which aresuperior to LCD screens in brightness, colour rendition, response timeand operating temperature range.

In known FEDS, electrons are released from a cathode by field emission(rater than by thermionic emission, as in the CRT) and acceleratedtoward an anode which is maintained at a positive potential typically ofseveral kV. The electrons impinge on the phosphor pixels which arethereby caused to luminesce, providing the display. To generate thefield needed for release of electrons, a matrix of switchable row andcolumn electrodes is typically provided, in addition to the anode, andin this way pixels can be individually addressed.

A particular problem has been found in providing a cathode whichexhibits field emission at electric field strengths which can beprovided in a practical display, without the need for unacceptably largepower and voltage.

Known FEDs typically utilize cathodes having on their surface an arrayof microscopic pointed elements known as Spindt tips, formed of Mo orSi. The tips are very sharp—having radii of the order of 20 nm—therebyproviding large local electric fields to cause field emission. This isnecessary because in the materials of such known cathodes the workfunction (the energy needed to release an electron from the cathode) isotherwise relatively high—or the order of 5 eV. These known cathodes arenot straightforward to manufacture and suffer from reliability problemsdue to erosion of the field tips.

An alternative approach which has been provided is to pride a cathodelacking Spindt tips but formed of material having low (or even negative)electron affinity. Electrons may be released from such a material byrelatively small electric field.

There is in almost all such field emission systems the need toelectrically condition the cathode before low threshold emission ispossible. Diamond like carbon (DLC) films have given high emission,particularly when doped with nitrogen. One such doped material hasprovided what is believed to be the lowest threshold reported at thepriority date. The nature of the bonding is thought to be an importantfactor with diamond-like sp³ bonds being appropriate for producing theenergy levels associated with a low electron affinity.

Current understanding of the main features of the emission processes isincomplete, but it is believed that the density of sp³ bands and thepresence of hydrogen are important. An alternative model is based on adual process which involves electron heating in the DLC conduction banddue to its internal electric field and emission over the relatively lowsurface potential barrier (electron affinity). Nitrogen acts as a donorfostering the formation of a high field depletion region. This highfield region promoters transfer of electrodes from substrate to film.

Although DLC film cathodes have hitherto been considered the mostpromising candidate, the result; have not been sufficiently impressivefor displays based on such materials to be considered an immediatereplacement for the CRT and LCD.

A first object of the present invention is to provide an improved fieldemission cathode.

It is desired that such a cathode should exhibit field emission whensubject to an electric field of a magnitude which can be created in apracticed display.

It is additionally or alternatively desired that such a cathode shouldexhibit stable field emission properties.

It is additionally or alternatively desired that such a cathode shouldbe capable of manufacture in suitable form for use in a field emissiondisplay.

In accordance with a first aspect of the present invention, there isprovided a field emission cathode comprising conjugated polymer waterforming a field emission surface.

The inventors have fortuitously (and most unexpectedly) discovered thatpolymer materials can be manufactured giving high electron emission.Polymer materials can be formed by known techniques into uniformcathodes, which may be large in area, and can be highly stable.Exclusion of oxygen is considered useful for the stability of thematerial.

Conjugated polymers typically have high density of free electrons. Mostpolymer films are p type with few free electrons; the substrates of thecathode can itself contribute electrons

Such materials are known for other applications in electronics, whichutilize semiconductor type properties of certain conjugated polymers.The usual applications proposed for conjugated polymers—eg. in lightemitting structures, photocopiers, photodetectors and thin filmtransistors—do not require the material to have a low work function, andit is believed that this property of such materials ha not hitherto beenutilized. The present inventors have found that some such polymericmaterial are capable of producing very high steady state field cessioncurrents with the threshold field needed to initiate field mission beingsmaller than for any other so far reported

It is especially preferred that the polymer material is a substitutedpolythiophene, and polyalkylthiophenes are particularly suitable. Poly-3-octylthiophene is currently the preferred martial.

The polymer material may take the form of a layer on a substrate. It isespecially preferred that the polymer layer is formed as a film withthickness of the order of 5 μm.

The polymer material is preferably spun from a liquid source or isevaporated in a vacuum onto a substrate techniques which can produce alarge area cathode. A light and economical cathode can be produced inthis way.

It in preferable ta the polymer material should have a low barrier toelectrons of the substrate on which it is formed.

The polymer material may be nitrogen with an electron donor material.The electron donor may be nitrogen (known to reduce the barrier toelectrons of the substrate of diamond-like-carbon). In fact, un-dopedpolymers have relatively low number of electrons but transport electronsfilm the substrate very efficiently. This leaves a greet deal of scopefor improvement by doping.

The polymer material may have envy levels which trap electrons servingto concentrate the electric field. In this way, field emission ispromoted. In a preferred structure, the polymer material forms a film ona substrate comprising microcrystalline silicon. The grain boundaries atthe polysilicon surface trap large numbers of electrons as the surfacebecomes more n type, and so are able to concentrate the field at thesepoints, promoting increased emission from the polymer film.

It is believed that in the material samples which led to the inventoryinitial discovery, voids observed on the material surface, believed tobe due to solvent evaporation, serve to create lug local electric fieldsat certain parts of the field emission surface and to promote fieldemission.

However, a great advantage of the use of polymer materials is that manyof them can be shaped by use of a mould. The term “mould” as usedthroughout this document must be understood to include any type ofprocess in which the polymer is shaped by contact with an appropriatelyformed tool which is then removed and “moulding” is to becorrespondingly constructed. By moulding the polymer's emission surfacecan be shaped such as to promote field emission, eg. by forming tipsthereon.

In accordance with a further aspect of td present invention, theme is afield emission display having a cathode in accordance with the firstaspect of the invention.

There are however other applications for the cathode.

At the low field strength which is sufficient to cause field emissionfrom the cathode according to the present invention, emitted electronsmay be insufficiently energetic to cause luminescence of a displayscreen. This problem is experienced when a phosphor screen is used.

Hence a preferred from of visual display device according to the presentinvention comprises a grid positioned with respect to the cathode suchas to be capable of causing field emission therefrom, an accelerationanode positioned beyond the grid and a luminescent screen, whereinelectrons are selectively emitted from the cathode under the influenceof the grid and then accelerated onto the screen with sufficient energyto cause it to luminescence by the acceleration anode.

In accordance with a third aspect of the present invention, there is amethod of fabricating a field emission cathode comprising forming alayer comprising conjugated polymer material on a subs the polymermaterial forming a field emission of the cathode.

The polymer material may be any of the polymer metals referred to abovewith respect to the first aspect of the invention.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:—

FIG. 1 is a graph of normalized or average field emission currentdensity (on a logarithmic scale of Acm⁻² on the vertical axis) againstapplied voltage for a cathode embodying the preset invention;

FIG. 2 a is a graph of normalized field emission current density from acathode according to the present invention in A cm⁻² a logarithmic scaleon the vertical axis against normalized applied electric field on thehorizontal axis, measured in volts per μm. Results are shown for throeanode-cathode spacings, the left-most line being for a spacing of 27 μm,the middle line for 47 μm and the right-most line for 130 μm;

FIG. 2 b is a graph of normalized field emission current density from acathode according to the present invention in A cm⁻² and a logarithmicscale on the vertical axis against applied anode voltage on thehorizontal axis. Results are shown for the three anode-cathode spacingsmentioned above with reference to FIG. 2 a;

FIG. 3 is a graph of field emission current from a cathode according tothe present invention in μA on the vertical axis against time in hourson the horizontal axis;

FIG. 4 shows band diagrams for a cathode according to the presentinvention comprising a P3OT layer on a p-Si substrate when subject to nobias (FIG. 4 a) and when subject to a biasing field from an Al anode(FIG. 4 b);

FIG. 5 shows band diagrams for a cathode according to the presentinvention comprising a P3OT layer on a n-Si substrate when subject to nobias (FIG. 5 a) and when subject to a biasing field from an Al anode(FIG. 5 b);

FIG. 6 is a band diagram a model for the field emission of electronsinto the vacuum from a cathode according to the present invention;

FIG. 7 represents a simple model of field emission from a cathodeaccording to the invention;

FIG. 8 schematically illustrated levels in the region of a void of thesurface of a cathode according to the present invention;

FIG. 9 is a graph of predicted field emission current (in a logarithmicscale as the vertical axis) against anode voltage on the horizontal axisassuming a simple one dimensional model for three differentanode/cathode separations:

FIG. 10 illustrated a step in the process of moulding a polymer layer ofa cathode according to the present invention, the cathode and mouldbeing seen in cross section;

FIG. 11 illustrates in cross section a structure of a display deviceconstructed in accordance with the present invention; and

FIG. 12 illustrates in cross section a further structure of a displaydevice according to the present invention.

A first specific embodiment of the invention comprises a field emissioncathode firmed as a thick (5 μm), spin-cast, nominally undoped polymer(regioregular poly-3-octylthiophene-P3OT) which is capable of high,stable electron emission. The observed threshold field (0.2MV cm-¹) isthe lowest so far reported among carbon based materials.

P3OT can be synthesized in pellets by the technique reported in Chen,T-A, Wu, X-M & Rieke, R. D.: Regiocontrolled synthesis of poly(3-alkylthiophenes) medicated by Riezo inc: their characteristics andsolid state properties, J. Am. Ch Soc. 117,233-244 (1995). Howeverduring a cared out by the inventors the material was purchased from acommercial source.

During trials, chloroform has been used as a solvent; dissolving 5 mg ofP3OT in 1 ml of chloroform resulted in films of thickness 5 μm.

In initial tests the solution was cast onto a pre-cleansed 1 cm×1 cmhighly doped n-Si substrate (resistivity 0.0030−0.01 Ω cm) in a cleannon-vacuum atmosphere. The optical Tauo bandgap was determined, at roomtemperature, to be −1.75 eV by ultraviolet-visible spectrophotometry(this bandgap is defined as the intercept of the gradient of a(αE)^(1/4) versus E plot with the E axis where a is the absorptioncoefficient and E the photon energy). In addition, a single broadabsorption maximum was obtained at −2.38 eV, which suggests that these“as synthesized” films are only slightly doped. Nuclear magneticresonance measurements indicate a regular “head-tail” arrangement of thealkyl side chains (which can be thought of as a measure of how orderedthe polymers are) of >90%. Such high regularity has only recently becomeavailable. It may not be essential for this application. Followingcasting the P3OT/n⁺⁺—Si devices were immediately placed in a vacuum ofpressure −10⁷ torr to dry for 24 hours.

No special processes we introduced to eliminate the effects of light orair during preparation of the films. Hence in these initial trials theywere invariably p type.

Field-emission experiments have been carried out using cathodes formedin the above manner in the flat plate configuration with an indium tinoxide coated glass anode. The separation between the anode and cathodewas varied between 27 μm and 130 μm using glass-fibre spacers. Thepressure in the vacuum was −2×10⁻⁷ torr. As with many carbon basedmaterials it was necessary initially to ramp the anode voltage to a highlevel to achieve field emission—ie. to an the film. FIG. 1 shows typicalcurrent-voltage (I-V) characters. Two regions are clearly identified(called here 1 and 2). Regions 1 a and 1 b represent the emission beforeconditioning and region 2 is after conditioning. A highly sable andreproducible current level was maintained after conditioning. Theconditioning field was always below 10V° m⁻¹ in order to avoid anyinfluence of vacuum breakdown phenomena, which can occur at fields above15V μm⁻¹ at the pressure used.

The results were found to be repeatable with or without the spacer beingin contact with the ITO anode. Raising the pressure caused the emissioncurrent measured at the anode to fall to zero. The emission current wasfound to scale with the area of the film rather than its perimeter,indicating that edge effects were not dominant.

The remarkable current density (J)-electric field (F) characteristics,with threshhold field of only −0.2V μm⁻¹ for a current density of 1 μAcm⁻² are shown in FIG. 2 a. FIG. 2 b shows the J-F characteristics as afunction of anode-cathode spacing. In addition a slope of 1.8 of the logI-log V plot indicates that a space-charge-limited current (SCLC) may beliming the emission process. The measured anode voltage versus gapseparation is shown in FIG. 2 a insert and indicates a nonlinearrelationship. This shows that surface and bulk charge properties may beimportant to the emission process.

The surface morphology of the film was examined by a scanning electronmicroscopy (SEM) both before and after field emission. There was notexplosive destruction of the film, which can be associated withdischarge current phenomena during field emission. The polymer filmsshow void-like features on the surface, of density −4×10 cm⁻² and ofsizes raging from 0.40 μm to a few micrometers.

The inventors also examined the stability of the emission current overtime. The emission characteristic recorded over a 16 hour period ofcontinuos emission is shown in FIG. 3. The initial current drops to 55%of its value in 2 hours, and thereafter remains stable. The sensitivityof the emission to a change in pressure was also examined: the emissioncurrent dropped to below 10 pA when the pressure rose to 10⁻³ torr.

An explanation of the current understanding of the physics of certainexemplary embodiments of the present invention is given below, but itmust be understood that the inventors do not thereby intend to limitthemselves to any particular model or theory explaining the operation ofthe present invention.

The band diagram of FIG. 4 for an aluminum anode predicts a very rapidrise of current which is observed with positive voltage on the aluminum.Space charge limited current kinks would be exposed with a high densityof donor-like traps in the bottom half of the P3OT energy gap. Al givesexcellent Schottky barriers on p-Si, with high on/off ratios.

The band diagram for the n substrate (FIG. 5) also predicts high forwardcurrents, this time due to electrons. Now there is significantdistortion due to the effects of trapped electrons in acceptor-liketraps in the top half of the band gap. Only a single trapping level isapparent, although a second one could be the cause of the saturation ofthe current at abut 10⁻⁴ A. A second interpretation is considered lesslikely: a low density of state at the edge of the conduction band could,in principle, explain the dominance trapping and hence the presence ofsuch distinct space charge limited effects.

The inventors conclude that the n-Si barrier is a reasonably efficientinjector of electrons, that the effective lifetime of electrons in thepolymer is, for minority carriers, surprisingly large. The high densityof traps in the top half of the polymer energy gap is significantlyhigher than in the lower half.

A further condition must be met for efficient field emission there mustbe an efficient mechanism for injection of electrons into the vacuum.The electron affinity of polymer is low but positive. There is,therefore, a brier to electrons at the surface of the polymer. Electronsconcentrate at this barrier, and most become trapped in the top half ofthe energy gap, close to the surface. There will therefore besignificant curvature of the conduction band, with the Fermi level nearto the conduction band c. This promotes electron emission via tunnelingacross the barrier (FIG. 6). Some of the electrons may travelballistically across the narrow surface space charge region where theyencounter a smaller tunneling distance. Further down the conduction bandwhere the concentration off electrons is higher the tunnel distance islower. FIG. 7 shows the proposed model in its simplest form. Theelectron density falls with distance from the silicon cathode, whilstthe field increases to maintain nE and therefor the current densityconstant. Standard analysis of space charge limited currents in thinfilms gives.Jx_(f)=(Θμε)(E₁ ^(2-E) ₂ ²)where x_(f) is the film thickness, Θ is the ratio of the free to totalelectron concentration in the film, μ is the free electron mobility,ε=ε_(o)ε_(r) where ε_(r) is the relative permissively of the polymer andε_(o) is the permittivity of free space. E₁, E₂and E_(vac) various fieldstrengths defined in FIG. 5.

The field within the surface of the polymer and the field in the vacuumare related by:εE₂=αε_(o)E_(vac)where α is the field intensification multiplier just outside the polymersurface.

As mentioned above, there is evidence from AFM and SFM pictures thatvoids exist in the surface of films prepared in the way descried above.It is proposed that these voids arm the ee of the field intensificationand that the electron current that flows has two components, one fromthe space charge limited behavior described above and a second due tofree space charge within the void itself. Such a current would also bespace charge limited obeying Child's Law. Both of these forms of spacecharge limited current have the form Jα V^(n) where n=1.5 in the case ofvacuum (Child's Law) and n=2 with solids. The rim of the void is foundto have a lip, shown schematically in FIG. 8, together with the regionswhere n=1.5 and n=2, The lip will act as an effective intensifier of thefield

Because the vacuum gap is much larger the film thickness the two formsof current must mix in this region and space charge in the vacuum due toChild's Law, in the region above the lips, will regulate the currentfrom the n=2 region. Similarly abrupt changes of potential will not bepossible between the lips and the adjacent space in the void—which is ofthe order of 1 μm in diameter or less. This will regulate current formthe void. These effects are not amendable to ID analysis, so we make asimplifying assumption that the two current densities are equal, as are,approximately, the two areas of emission. The resulting current voltageplots are shown in FIG. 9 for the three vacuum gaps used in the fieldemission experiment. Although the plots accurately predict the slope of1.8 for all three us, as well as for a range from 10V to 1 KV (notshown), there is a discrepancy in the thickness dependence.

Strictly speaking one would expect the simple analysis provided here toonly apply for vacuum gaps comparable in size to the thickness of thepolymer film. Extension to greater gap is the subject of 2D modeling.

The inventors have speculated that the voids in the polymer film may becreated by solvent evaporation during the manufacturing process. Inworking displays an alternative method of creating the necessarymorphology is considered desirable and to this end the polymer film maybe shaped against a mould. In such a process, the polymer material maybe cast or centrifugally spun and thereby distributed over thesubstrate. A female mould, seen at 100 in FIG. 10, then pressed againstthe upper surface of the polymer 102. In FIG. 10, the substrate is seenat 104.

Polymers can be well suited to moulding processes.

In the illustrated embodiment, the mould is shape to form in the polymera number of emission tips 106.

Display devices embodying the present invention are illustrated in FIGS.11 and 12.

In the simplified FED device structure illustrated in FIG. 11 the fieldemission cathode is a flat film 2 of polymer material disposed on aconducting substrate 3 maintained at low electrical potential. An anode4 is provided in front of the cathode, and bears pixels 5 of a materialwhich emits light when stuck by energetic electrodes, to produce thedisplay. The anode may consist of a light emitting phosphor on glass.Between the anode and the code is a grid 6 which can be selectivelypositively charged. The space 7 between the anode and the cathode is avacuum.

In operation of the selected regions of the grid 6 are chargedpositively (relative to the cathode) producing the field necessary forfield emission of electrons from corresponding regions of the cathode.These electrons are accelerated through the grid by the electric fielddue to the anode 4, and strike the anode causing it to cunt light inselected regions of the screen.

A more detailed illustration of a suitable device structure is given inFIG. 12. Here the cathode is formed by a shaped polymer layer 52 on asubstrate 54 which may be of glass. Spacers 56 separate the cathode froma conducting grid 58, penetrated by apertures 60 through which emittedelectrons can reach a screen forming the anode and comprising aluminescent phosphor anode 62 on a glass substrate 64. Here it is thegrid 59 which modulates the field emission (being positively chargedrelative to the cathode to the low voltage needed to produce fieldemission with the type of cathode described herein). The anode 62 ismaintained at a higher voltage to accelerate the electrons sufficientlyto cause the phosphor to luminesce with a required intensity.

1. A field emission apparatus comprising: a field emission cathodecomprising a polymer material forming an exposed field emission surface;and an anode separated from said field emission cathode such as to becapable of causing field emission therefrom.
 2. The field emissioncathode of claim 1 wherein said polymer material is a conjugate polymermaterial.
 3. The field emission cathode of claim 2 wherein saidconjugated polymer material is a substituted polythiophene.
 4. The fieldemission cathode of claim 2 wherein said conjugated polymer material isformed as a polymer layer on a substrate.
 5. The field emission cathodeof claim 4 wherein said polymer layer is formed from a polymer solutionincluding a solvent, which is distributed on said substrate, saidsolvent being evaporated to leave behind said polymer layer.
 6. Thefield emission cathode of claim 5 wherein said solvent is evaporatedunder vacuum.
 7. The field emission cathode of claim 5 wherein a surfaceof said polymer layer includes voids which are formed by solventevaporation.
 8. The field emission cathode of claim 7 wherein saidsurface of said polymer layer is shaped by use of a mould.
 9. The fieldemission cathode of claim 8 wherein said moulded surface of said polymerlayer comprises a plurality of projections which promote field emission.10. The field emission cathode of claim 2 wherein said conjugatedpolymer material is doped with an electron donor material.
 11. A fieldemission apparatus comprising: a field emission cathode comprising aconjugate polymer material forming an exposed field emission surface;and an anode separated from said field emission cathode such as to becapable of causing field emission therefrom, wherein said conjugatedpolymer material comprises a polyalkylthiophene.
 12. The field emissioncathode of claim 11, wherein said conjugated polymer material comprisespoly-3-octylthiophene.
 13. The field emission cathode of claim 11wherein said conjugated polymer material is formed as a polymer layer ona substrate.
 14. The field emission cathode of claim 13 wherein saidpolymer layer is formed from a polymer solution including a solvent,which is distributed on said substrate, said solvent being evaporated toleave behind said polymer layer.
 15. The field emission cathode of claim14 wherein said solvent is evaporated under vacuum.
 16. The fieldemission cathode of claim 14 wherein a surface of said polymer layerincludes voids which are formed by solvent evaporation.
 17. The fieldemission cathode of claim 16 wherein said surface of polymer layer isshaped by use of a mould.
 18. The field emission cathode of claim 17wherein said moulded ace of said polymer layer comprises a plurality ofprojections which promote field emission.
 19. The field emission cathodeof claim 11 wherein said conjugated polymer material is doped with anelectron donor material.
 20. A field emission display comprising: afield emission cathode comprising a conjugated polymer material forminga field emission surface; a first anode separated from said fieldemission cathode such as to be capable of causing field emissiontherefrom; a second anode positioned beyond said first anode; and aluminescent screen, wherein electrons are selectively emitted from saidfield emission cathode under the influence of said first anode thenaccelerated onto said screen with sufficient energy to cause it toluminesce by said second anode.
 21. A method of fabricating a fieldemission cathode comprising the step of forming a polymer layercomprising conjugated polymer material on a substrate, said polymermaterial forming an exposed field emission surface of said fieldemission cathode.
 22. The method of fabricating a field emission cathodeof claim 21 further comprising the steps of: distributing a polymersolution including a solvent on said substrate, and evaporating saidsolvent to leave behind said polymer layer.
 23. A method of fabricatinga field emission cathode comprising the steps of: forming a polymerlayer comprising conjugated polymer material on a substrate, saidpolymer material forming an exposed field emission surface of said fieldemission cathode; and distributing a polymer solution including asolvent on said substrate, and evaporating said solvent to leave behindsaid polymer layer, wherein said solvent is evaporated under vacuum. 24.The method of fabricating a field emission cathode of claim 23 furthercomprising the step of shaping the surface of the polymer layer by useof a mould.